1. Field of the Invention
The present invention relates to a differential apparatus for an automobile, more particularly, to an art which can be effectively applied to a differential apparatus having a limited slip differential mechanism.
2. Related Art
A power train of an automobile for transmitting an engine output to driving wheels comprises various devices such as a start clutch, a transmission, a propeller shaft, and a differential apparatus. A rotational driving force transmitted from the engine to the differential apparatus is distributed to each driving wheel depending on the driving condition of the automobile.
A differential apparatus includes a differential mechanism which enables a smooth driving of the automobile by absorbing a rotational speed difference between the right and left driving wheels when the automobile negotiates curves. The differential mechanism comprises a gear case which is driven by a rotational driving force from the engine, two side gears housed in the gear case and connected to the driving wheels through drive shafts, and two pinion gears which rotate around a pinion shaft fixed on the differential case and mesh with the two side gears. In the gear case, since the pinion gears rotate around a pinion shaft depending on the rotational speed difference between the two side gears, the rotation of the side gears and the driving torque transmitted by the side gears are increased or decreased depending on the rotational resistances applied on them. The differential apparatus can be used to distribute the rotational driving force of the engine to the right and left driving wheels of front and rear axles, and to distribute the rotational driving force of the engine to the front and rear axles in the transmission.
Since the differential apparatus distributes the rotational driving force depending on the driving resistances applied on the driving wheels, when one of the driving wheels slips on a snowy or icy road, almost all of the driving power is distributed to the driving wheel which is slipping on the road. As a result, the total rotational driving force for moving the automobile forward is inevitably reduced.
Therefore, there has been developed a differential apparatus having a limited slip differential mechanism for limiting the differential rotation between two side gears to distribute the rotational driving force to the driving wheel which is not slipping on the road. Such limited slip differential mechanisms are divided into multiple types, including a mechanical friction type which controls a differential rotation using a mechanical friction torque, and a rotational-speed-sensitive type which controls a differential rotation using a viscous fluid which generates a fluidic resistive torque depending on a rotational speed difference between the differential case and the side gears.
In a friction-type limited slip differential mechanism, the side gears and the pinion gears are formed as helical gears, such that the axial component of a meshing reaction force of the helical gears generates a frictional force between an end surface of the pinion gears and a differential case, and the radial component of the meshing reaction force generates a frictional force between tooth tops of the pinion gears and the differential case, to limit the differential function of the differential apparatus (see Japanese Patent Laid-Open Publication No. 101742/1994 (page 3, FIG. 1)).
In another friction-type limited slip differential mechanism, side gears and pinion gears are formed as bevel gears, and tapered rings are interposed between the side gears and a differential case, respectively, such that the axial components of a meshing reaction force of the bevel gears are converted into larger frictional forces between end surfaces of the side gears and the differential case, to limit the differential function of the differential apparatus (see Japanese Patent Laid-Open Publication No. 82680/1999 (page 3, FIG. 1)).
In a rotational-speed-sensitive-type limited slip differential mechanism, a side gear is connected to a differential case by a viscous coupling, such that a shear resistance of a viscous fluid in the viscous coupling is generated depending on a rotational speed difference between the side gear and the differential case, to limit the differential function of the differential apparatus.
However, in the above-described limited slip differential mechanisms, the meshing reaction force and the shear resistance of the viscous fluid are the same in a drive mode, in which the rotational driving force is transmitted from the pinion gear to the side gear, and in a coast mode, in which the rotational driving force is transmitted from the side gear to the pinion gear. As a result, it is difficult to set a difference in the differential limiting forces between the drive mode, in which a driver of the automobile depresses an accelerator pedal, and the coast mode, in which the driver releases the accelerator pedal to apply an engine brake.
Further, if the differential limiting force in the drive mode is reduced, the loss of a driving power deteriorates the acceleration performance of the vehicle when the vehicle starts or exits curve. Although the acceleration performance in the drive mode can be raised by increasing the differential limiting force in the drive mode, this, in turn, deteriorates the turning performance when the vehicle slows down and enters the curve due to the increase of the differential limiting force in the coast mode. Thus, it is difficult to raise the performance of the vehicle both in the drive mode and in the coast mode with the differential apparatus which generates the same differential limiting force in the drive mode and in the coast mode.